Green Gabbro

Every time there’s an earthquake in the Midwest, my mother emails me, just in case I want to move back home to study it. So that’s how I heard about yesterday morning’s earthquake in Illinois – a bit less exciting than waking up to it, but that’s fine with me.

This is not earthquake enough to reverse the geoscientific brain drain from the Midwest to the West Coast (there’s also the weather to think about)… but for an ostensibly stable part of the continental interior, thousands of miles from the nearest plate boundary, Illinois and Indiana have seen an awful lot of seismic action over the past 12,000 years:

i-1351f9a4c7febcf49aaa05be16cdcbf3-wvsz-paleoseismicity.gif
Estimated locations of prehistoric earthquakes in Illinois and Indiana, from Obermeier (1998). Stars are historical earthquakes (1804-1992) of magnitude 5 or greater, dots are magnitude 4.5-5, and the light gray plus signs are weer than 4.5. The prehistoric earthquakes on this map were found by digging around stream banks, looking for preserved sand boils and other evidence of liquefaction.

At the bottom of this map are the famous New Madrid earthquakes of 1811 and 1812, which made the Mississippi run backward and rang church bells in Boston. Are they connected to yesterday morning’s event?

The short answer is yes and the long answer is sort of, but it’s not as simple as you are probably imagining.

Six hundred million years ago, the former supercontinent of Rodinia was in the late stages of a nasty breakup. Laurentia (the tectonic unit now known as North America) actually tried to split apart in several places, but somehow managed to hold itself together. But the experience left scars.

We are fairly certain that one of the scars from that breakup lies underneath the Mississippi River near the town of New Madrid, Missouri. Six hundred million years ago the area would have looked much like the East African Rift Valley does today, like it’s almost ready to give birth to a brand new ocean.

These failed rifts are weak spots. Like an old injury when bad weather approaches, they will pop and crackle whenever the tectonic weather is favorable.

i-0a441d7f7fe64495ac3e05f241d08f29-iapetan-aulacogens.jpg
Four arms of the failed rift zone? From Nelson 1990, who modified it from Braile et al. 1982.

So that’s New Madrid. But yesterday’s earthquake wasn’t in the New Madrid Seismic Zone, it was in the Wabash Valley Seismic Zone. The WVSZ is roughly coincident with the “Southern Indiana arm” drawn in dotted lines on the above figure. Those lines were drawn based on gravity and magnetic data (there is something heavy and made of iron down there, which might be basalt left over from the rift). The WVSZ also contains faults that are approximately the same age as the failed rift. However, there’s not much structural or seismic evidence to support the idea that they’re one and the same rift valley.

So, the earthquake in Illinois yesterday was probably related to the New Madrid earthquakes, inasmuch as both events occurred on weak spots left over from a particularly violent period in North America’s tectonic history. But they’re not really part of the same structure, and they’re certainly not on the same fault.

Today’s event was probably too small to have affected the chances of a big quake at New Madrid one way or the other.

References

  • S.F. Obermeier (1998) Liquefaction evidence for strong earthquakes of Holocene and latest Pleistocene ages in the states of Indiana and Illinois, USA. Engineering Geology 50 227-254, doi:10.1016/10.1016/S0013-7952(98)00032-5
  • W. John Nelson (1990) Comment on “Major Proterozoic basement features of the eastern midcontinent of North America revealed by recent COCORP profiling” Geology 18 378. (See also Pratt et al., Reply, and the original 1989 article by Pratt et al.)
  • Won-Young Kim (2003), The 18 June 2002 Caborn, Indiana, Earthquake: Reactivation of Ancient Rift in the Wabash Valley Seismic Zone? Bull. Seis. Soc. Am. 93, 2201-2211, doi:10.1785/0120020223

Comments

  1. #1 Chris
    April 19, 2008

    Thanks for the article. Mid-continental earthquakes have always interested me, but I’ve never looked into them. And I was totally unaware of the failed North American rift, that’s just super cool!

  2. #2 Alice
    April 19, 2008

    Thanks for the explanation! And the images. Now I just have to work out how I am sure I remember it happening at 4:30ish EDT but it actually happened at 5:30ish EDT… :-)

  3. #3 chezjake
    April 19, 2008

    Thanks for the clear, easily understandable explanation. I really like your analogy “Like an old injury when bad weather approaches, they will pop and crackle whenever the tectonic weather is favorable.”

    Good stuff!

  4. #4 Susie
    April 19, 2008

    You know that feeling where you read something really interesting about a topic you don’t know a lot about, and are so fascinated that you toy with the idea of dropping everything and going and learning about that topic? That would be the feeling I’m having at the moment. I suppose it helps that this is the area I’m from originally, so there’s something extra compelling about imagining its geological history and evolution.

  5. #5 ScienceWoman
    April 19, 2008

    This is a great clear explanation. Thanks for putting it together.

  6. #6 Greg Laden
    April 19, 2008

    Nice job.

  7. #7 Greg Laden
    April 19, 2008

    OK, this might be a philosophical question more than a geology question, but lets have a go at it:

    Is the seismic activity in this buried ancient rift system (broadly speaking, not in reference to this recent earthquake) a result of very low level continued tectonic activity in this area? (I think not); Continued adjustment to that early activity (much like changes in the sea surface near the Chesapeake crater continue tens of millions of years after the impact) (seems possible)?; or occur here because there is structural discontinuity or something at this location so that stresses happen to get relieved here? You seem to favor the last given your “old injury” analogy, but is that really what you are saying?

  8. #8 The Christian Cynic
    April 19, 2008

    Great post. I assumed that it was from the New Madrid zone when I woke up to it (how very disconcerting an experience, I might add, for a lifelong Illinois resident where earthquakes seem about as likely an occurrence as hurricanes). I wasn’t even aware of the different “branches” of the rift zone, so that’s very cool to know as well.

  9. #9 Maria Brumm
    April 19, 2008

    Greg: Yes, I’m really saying that stresses happen to get relieved in places where there is a conveniently-oriented weak spot. I haven’t done any serious looking-in to the matter since an undergrad term paper, but back then my favorite hypothesis was that a lot of the activity at New Madrid is actually due to melting of the Laurentide ice sheet.

    “Tectonic activity” is not exclusive to plate boundaries, so technically the seismic activity here is a low level of continued tectonic activity.

  10. #10 Kim
    April 19, 2008

    The first motion beachball is consistent with some kind of near-vertical fault on the southern Indiana arm, too. It’s approximately vertical, strike-slip, with one possible fault about parallel to that old rift on your map, and compression should be east-west.

    Actually, it would make a good question for a final exam… heh.

  11. #11 Andrew
    April 21, 2008

    Greg, that’s a good question, and Maria’s favored hypothesis is still in the lead. My analogy is that the failed rift (or aulacogen, for us lovers of the jargon) is like a creaky floorboard, and the long cycle of ice ages has acted like someone pacing up and down the floor, making it creak. The floor is in no danger of breaking, it just creaks.

    The thing is, some modelers have argued that the handful of great quakes there since the glacier last left, about 10,000 years ago, may be all the creaks we should expect, and that 1811-12’s quakes were the last there will be until the glaciers return in a few millennia. The argument is fairly made, but it reminds me too much of the people who, after 1906, argued that the San Francisco made us all safer. Technically it did, by creating a stress shadow across the Bay area, but it didn’t make us SAFE. Exactly how safe the Midwest is is a lively question among the handful of Midwestern seismologists (and those who love them). The magnitude of expected shaking translates to dollars, after all.

  12. #12 YetAnotherKevin
    April 23, 2008

    Um. Is there a “Regional Moment Tensor Solutions for Dummies?” Also, if this is the same structure described in “Crossing the Craton” I thought it was further north.

  13. #13 Kim
    April 23, 2008

    YetAnotherKevin: Vince Cronin from Baylor has a good handout here. The USGS has a much shorter explanation in its earthquake FAQ, but it’s saturated with jargon. I thought there was a short version in the geoblogosphere, but I think there have just been references to Vince Cronin’s handout.

  14. #14 Kim
    April 23, 2008

    And Andrew Alden has a good explanation here.

  15. #15 Maria
    April 24, 2008

    YAK: It’s been a while since I’ve read “Crossing the Craton”, but it may have been talking about a different – and much older – failed rift in Iowa.

  16. #16 klgregonis
    April 25, 2008

    Does anyone recall a science fiction story about the failed rift coming to life and creating an inland sea? I think it ends with a philosophical comment about how it ultimately benefited the US, and mentions Denver as a major shipping port. Unfortunately, I think it was in a collection I gave away many years ago.

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